It has been known for many years that the free-radical-initiated copolymerization of monounsaturated olefins is truly satisfactory only with monomers which have high "e" values. Monounsaturated olefins are molecules containing only carbon and hydrogen atoms and without aryl groups and without any functional group, such as halogen, carboalkoxy, and the like, and further containing only one carbon-carbon double bond. An "e" value of a monomer is a measure of the polarity of the monomer, or more accurately the electron-attracting or electron-donating attributes of the substituents on the monomer. The "e" values are calculated numbers, but are based on copolymerization data. They range from -1.58 for vinyl t-butyl ether, a monomer with electron-donating functionality to about 0 for a non-polar material with no electron-donating or -withdrawing groups (ethylene) to +2.25 for a highly polar material, such as maleic anhydride and the like, which usually contain two strongly electron-withdrawing groups. The Q-e calculations are well-known to the polymer art and are found, inter alia, in "Polymer Handbook, 3rd edition, John Wiley and Sons, 1989, page II-267. In order for a monounsaturated olefin to effectively copolymerize with a polar monomer, the "e" value of the polar molecule must be at least about +1.2.
Relatively few polar monomers have high enough "e" values (at least +1.2) to form alternating copolymers with non-functionalized monoolefins. Useful polar monomers, which usually contain two strongly electron-withdrawing groups, include maleic anhydride, maleimide, N-substituted maleimides, a-cyanoacrylate esters, vinylidene cyanide, and the like. Of these, maleic anhydride is preferred for cost, and because many of the uses for the copolymer involve the copolymerized anhydride group, more often in its hydrolyzed form as the acid or salt.
It is further known to be far more favorable for terminally unsaturated monoolefins, such as ethylene, propylene, isobutylene, 2,4,4-trimethylpentene-1, 4,6-dimethylheptene-1, and the like, to copolymerize with polar monomers having "e" values in the above range than for monounsaturated olefins with internal or non-terminal unsaturation, such as 2,4,4-trimethylpentene-2, 4,6-dimethylheptene-2, and the like.
In many instances, a terminally-unsaturated olefin can be obtained in a pure form, but in others, such as from acid-catalyzed oligomerization of low molecular weight olefins such as propylene and isobutylene, the terminally-unsaturated olefin formed is mixed with an internally-unsaturated or non-terminally unsaturated double bond. The mixtures of olefins are extremely difficult to separate by inexpensive physical means, such as distillation. In the presence of a catalyst, such as a strong acid, a purified olefin will revert to an equilibrium mixture of olefins, the equilibrium value being specific to each olefin. For example, the equilibrium value for commercial diisobutylene is ca. 76% 2,4,4-trimethylpentene-1 and 24% 2,4,4-trimethylpentene-2.
The problem with copolymerizing polar monomers with mixtures of olefins is that the relative concentration of the unpolymerizable non-terminal olefin isomer increases during the copolymerization reaction. The presence of the unreacted olefin in the product reduces the desired solids level in the product. The increasing concentration of the unpolymerizable non-terminally unsaturated olefin monomer in the copolymer product requires it typically to be stripped from the reaction and burned.
The mixture of monounsaturated olefins may be removed during or after the copolymerization. The removed olefins may then be subjected to a separate isomerization reaction to re-isomerize the mixture back to the initial equilibrium amount of the terminally-unsaturated olefin. The re-isomerized olefin mixture is then returned to take part in the copolymerization reaction. Apart from higher capital equipment costs, the external isomerization is difficult to coordinate with the on-going copolymerization in terms of rates of polymerization and feed schedules.
U.S. Pat. No. 4,151,336 describes a conventional diisobutylene olefin mixture/maleic anhydride copolymerization process which requires ( Examples 1 -4) high ratios of diisobutylene to maleic anhydride, produces a relatively low solids product and requires the removal of the unreacted diisobutylene, which at the point of removal is high in non-terminal olefin content.
The copolymer of "diisobutylene" with maleic anhydride is a commercially useful product. Industrial uses include use as components of preprint overlayer varnishes, laminating inks, preprint inks, paper sizings, components of water treatment additives, detergents, and the like. This copolymer is actually the copolymer of the terminally unsaturated diisobutylene isomer 2,4,4-trimethylpentene-1 with maleic anhydride. For many years the industry has sought means to effectively incorporate most or all of the diisobutylene charged (generally about a 3/1 mixture of the 1- and 2-isomers),to yield a higher solids product with less residual material to remove, and without the need for off-line isomerization.